1. Field of the Invention
The invention relates to a nitride sintered body having a high thermal conductivity and a high reflectance in the visible light region and a method for manufacturing thereof.
2. Description of the Background Art
Monochromatic light emitting diodes (a light emitting diode is hereinafter referred to as LED) emitting light in colors such as red, green and blue, and a white LED have been introduced commercially, which has increased applications of light emitting elements using LED to a large extent.
For example, an LED emitting blue light using a GaN based compound semiconductor is fabricated and put into practice in a way such that an insulating sapphire is generally adopted as a substrate, p side and n side electrodes are formed on the front surface side of a compound semiconductor stacked on the substrate, which is used as a so-called flip chip type light emitting element surface-mounted on the electrode face thereof. Since sapphire of the substrate is light transmissive in such a flip chip type light emitting element, the sapphire substrate is mounted on a substrate, being directed to a light emitting direction side and the surface of the sapphire substrate can be used as a main light take-out surface. Recently, a light emitting element chip has been mounted on a substrate of equipment, and as a useful light emission source, a compound light emitting element has been employed that is obtained by mounting the light emitting element chip, for example, on a submount element used for electrostatic protection with a Zener diode.
Such a compound light emitting element has a structure that a flip chip type of blue light emitting element is conduction-mounted on a submount, which is conduction-mounted on a mounting substrate incorporated in electronic equipment or the like. Conventionally, a silicon substrate has been used as a submount, while there has been a problem that a compound light emitting element is reduced in luminance thereof since the silicon substrate absorbs light in the wavelength range of from 450 nm (blue) to 560 nm (green) emitted from an LED.
A proposal has been offered in Japanese Patent Application Laid-Open (JP-A) No. 2003-60243 of a structure, as a compound light emitting element without such a problem, that a mounting surface of a light emitting element is made of a white insulating material such as alumina. In recent years, however, a problem has occurred that a generated heat quantity from an LED increases in company with progress toward high luminance, thereby raising an element temperature. When an element temperature rises to exceed an allowable value, a light emission wavelength is shifted to the longer wavelength side, a chroma saturation decreases because of a broader half value width of a wavelength distribution, and luminance is reduced, thereby lowering characteristics of the element. A necessity arises for heat to be radiated by a heat radiating member through a submount in order to prevent a temperature of such an element from rising, whereas an alumina substrate that has been conventionally employed as a submount material in JP-A 2003-60243 has a problem that a thermal conductivity thereof is as low as about 20 W/m·K, thereby disabling effective heat radiation to be achieved.
Though such a problem is thought to be able to be solved by constituting a submount with an insulating material high both in thermal conductivity and light reflectance, no insulating material meeting such a requirement has been found. For example, an SiO2—Al2O3—MgO—ZeO2—Cao based ceramic with a specific composition has been known as a white ceramic other than alumina (see JP-A No. 2004-152952). Since the ceramic includes Al2O3 as a main component (96.25 wt %), however, a thermal conductivity thereof is as low as expected.
Although a boron nitride sintered body has been known as expressing a white color, the thermal conductivity of the boron nitride sintered body commercially available is about 20 W/m·K, of which range is same as that of alumina. On the other hand, nitride sintered bodies including aluminum nitride sintered body or silicon nitride sintered body have been known as an insulating material high in thermal conductivity, whereas a nitride sintered body that has been conventionally known has a problem in the aspect of a light reflectance. For example, an aluminum nitride sintered body is of a tone of light transmissive gray color (refer to the comparative example 6 of present application) and a silicon nitride sintered body has a gray or a black color.
Note that in JP-A No. 2004-152952, it is described that a content of Er2O3 is preferably in the range of 1 to 10 wt % relative to a total weight of aluminum nitride sintered body for getting a reflectance of light with a wavelength in the range of 400 to 700 nm is 80% or more. There is no disclosure of examples corresponding to the description, however, and it has been confirmed that even sintering of a composition for an aluminum nitride sintered body having a content of Er2O3 in the range of 1 to 10 wt % relative to a total weight thereof cannot produces a white sintered body (see the comparative example 5 of the present application).